The manufacture of microelectromechanical systems (“MEMS”) is currently impaired not by the microelectronic side of MEMS, but rather by the micromechanical side. Microelectronics is now a mature art—and primarily a two-dimensional one. Micro-mechanics, however, is suffering from a prolonged adolescence, its development having been hindered by the attempts of its parents to make it “wear its sibling's old clothes.” While microelectronic components are substantially two dimensional, it is desired that micromechanical components have significant features in the third dimension. Despite this, the majority of micromechanical devices are produced using the inherently 2D methods of lithography-based microelectronics fabrication. Some of these methods have been tailored to produce structures with significant thickness, for example, LIGA (lithography, electroplating, and molding), and thick UV photolithography for electrodeposition, bulk silicon micromachining, silicon direct bonding, etc., but the drawbacks are still present—namely, the reliance of mask sets and the associated labor when it comes to changing device design (which requires another mask set). Additionally, the three-dimensional complexity attainable with such methods is limited; the number of layers rarely exceeding a few dozen. Finally, these processes must be performed in a cleanroom facility, representing a substantial capital investment for a would-be micromachinist.
We have developed an electrochemical printing system that seeks to remedy the micromachinist's plight. The system is capable of producing two-dimensional multimaterial patterns. In its present form, it is ideally suited for custom bump-plating for high-density interconnect flip chips, as well as printed circuit board metallization and repair. Extending the method to the third dimension requires repeated printing of patterned layers comprised of functional-material along with a sacrificial support material. The ability to print multilayer patterns will have a significant impact on both prototyping and direct custom manufacturing of MEMS devices. A method of using the system for custom three-dimensional packaging is also described.
The MEMS Exchange (www.memsnet.org) cites the two main challenges facing the microelectromechanical systems (“MEMS”) community as—(1) insufficient access to MEMS fabrication facilities; and (2) an entanglement of the MEMS design process with the inherent complexities of current fabrication methodologies. Both of these issues stem from the following fact that conventional mask-based microelectronic fabrication processes are still the predominate means of MEMS prototyping.
These problems lie predominately on the micromechanical side of MEMS. While the mask-based fabrication of essentially two-dimensional microelectronic components is a well-established art, a flexible method for producing the arbitrary 3-D geometries ideal for micromechanical components has yet to be found. This sought-after process would ideally be capable of tailoring not only the geometry, but also the material properties in three dimensions, creating parts from so-called functionally graded materials (“FGMs”). The process must also be compatible with existing IC fabrication methods, especially in regard to the required processing temperatures. In lieu of developing such a tool, the MEMS community has thus far made do by adapting existing mask-based microelectronics fabrication techniques to meet some of the above demands. Specifically, significant z-dimensionality has been achieved using mask-based methods by the following strategies—(a) fabricating consecutive layers one on top of the other (e.g., surface silicon micromachining); (b) bonding individual layers after their respective fabrication (e.g., silicon-direct bonding); and (c) producing substantially thick individual layers, either by additive (e.g., LIGA and UV thick-resist lithography) or subtractive (e.g., bulk silicon micromachining) processes.
Tethering the process to lithographic masks, however, limits the number of layers that can feasibly be incorporated in the micromechanical structure, thus constraining design freedom and three-dimensionality. Design iterations are belabored by the need for a new mask or mask set for each alteration, prolonging the design-test phase of a MEMS product's lifecycle. Furthermore, the mask dependency typically requires that fabrication be conducted in a cleanroom facility, which represents a substantial cost to the designer.
Alternatives to mask-based micromachining are currently being developed, most of which are seeking to embody the tenets of “Solid Freeform Fabrication” (“SFF”). SFF has its origins in Rapid Prototyping, which was defined in the Rapid Prototyping Report, October 1992, as:                The fabrication of a physical, three dimensional part of arbitrary shape directly from a numerical description—typically a Computer Aided Design model, by a quick, highly automated and totally flexible manufacturing process.        
One of Rapid Prototyping's central goals is to reduce the time it takes to fabricate a testable device, thus expediting product development. Due to the limited materials available for most rapid prototyping processes, these tests are typically limited to form-fit testing. If the above definition is further extended to producing functional devices with the desired geometry AND material properties (the ideal of which are devices made of functionally graded materials), it becomes SFF, (alternatively referred to as “Direct Fabrication” and/or “Rapid Manufacturing”). Whatever the title, such a process applied to MEMS and available at a comparatively modest cost would give a lithography-wearied MEMS designer reason to rejoice.
The basis behind solid freeform fabrication is layered fabrication, building cross-section upon cross-section until the entire device is constructed. SFF is similar in this regard to mask-based microfabrication; however, this is where the similarity ends. SFF builds the layers rapidly, enabling high resolution in the z-direction and flexibility, producing the layers by fully automated computer control. It is therefore much like desktop printing extended to a third dimension.
Among the techniques being employed to direct-manufacture FGM parts is “3D Printing,” which forms each 2D cross-section of the device by selectively applying liquid binder droplets roughly 100 μm in diameter to a layer of powder. The composition can thus be varied by using different binders in much the same fashion as different inks are applied in inkjet printing. The finished parts, however, can be fragile and porous. Additionally, it may be difficult to remove the excess/nonbound powder from the cavities within the part. Alternately, a CO2 laser can be used to selectively sinter the powder at desired locations, as in Selective Layer Sintering. This process suffers from disadvantages similar to those of 3D Printing, however, in that the resulting part is porous. An additional complication, owing to the necessary heating of the powder, is the potential for thermal distortion of the part. Other processes, which do not involve powder layers, deposit individual droplets of build material to build the device. Shape Deposition Manufacturing does this by depositing 1-5 mm diameter drops of molten metal using a process called “microcasting.” Particular relationships must exist between the sacrificial/retained and retained i/retained i+1 materials' melting points and thermal conductivities in order for both (a) high strength bonding (via melt alloying) to occur between retained materials of different composition, and (b) easy removal of the sacrificial material after the build-up phase. A similar process is Ballistic Particle Manufacturing. The drops in this process are issued from a thermal spray nozzle and thus can have smaller diameters (50-100 μm), but must be selected from a more limited range of materials. One of the more promising FGM-SFF techniques is Laser Assisted Net Shaping, where several jets of metal particles are directed at a high-power laser's focal point. The laser heats the metal powders and causes them to melt and stick onto the substrate, with molten pool diameters ranging from ½ to 5 times the spot size of the laser. Composition gradients are obtained by varying the respective jets' flow rates during the deposition. A disadvantage associated with all processes involving deposition of molten or uncured materials, however, is the build-up of residual thermal stresses, which can cause warping and delamination in the finished product.
Among the techniques employing electrochemical reactions to direct-manufacture parts is the Electrochemical Fabrication process. This process has yet to sever all ties with masks, making it somewhat inflexible; however, its “instant masks” are far more flexible than ordinary lithographic masks. Electrochemical Fabrication applies these compliant masks directly to the anode (counter electrode) that drives the electrodeposition. An assembly line like process moves the substrate (working electrode) underneath a succession of such masks, which are each pressed to the substrate's surface during electroplating, thus depositing a metallic structure true to the pattern of the instant mask. The mask stamp is then lifted and the deposit backfilled with a second material, and planarized before proceeding to the next instant mask. Lastly and most similar to our disclosed invention, is the localized electrodeposition method of Hunter and Madden. This method employs a sharpened electrode, positioned very near the substrate, to drive an electrodeposition reaction in a confined region immediately beneath the electrode's tip. Their demonstrated embodiment relies on diffusion and migration to deliver the depositing species to the reaction zone and moving the tip in three dimensions results in the creation of a three-dimensional line of nickel. The associated patent (U.S. Pat. No. 5,641,391), however, includes an alternative embodiment wherein a nozzle provides forced convection at the reaction zone.
We propose herein a rapid and flexible method for electrochemically producing two-dimensional multimaterial patterns directly from a computer using bitmap graphics files as the exchange medium. The invention controls the size and location of localized electrochemical reaction zones to produce or remove dots of different materials—a plurality of such dots results in a two-dimensional pattern. The method can be extended to produce complex three-dimensional objects by creating multiple two-dimensional patterns on top of one another and optionally selectively dissolving at least one material from the finished “stack” to give a three-dimensionally complex part.